Method and apparatus for tuning a multiturn induction heating coil

ABSTRACT

In an induction heating device including a multiturn induction heating coil and at least one conductor extending from the coil to a power supply, which conductor is closely adjacent the outer periphery of the coil, there is provided means for adjusting the spacing between the conductor and the coil to tune the coil with an alternating frequency power supply.

PATENTEDMAR 14 I972 SHEET 1 OF 2 INVENTOR. ROBERT J. KASPER M W g M,

ATTORNEYS PATENTEDMAR 1 4 m2 3, 649,804

O CWM L 26 Jim 42 8 INVENTOR.

ROBERT J. KASPER BY Mew, 744m 8 Bad;

ATTORNEYS METHOD AND APPARATUS FOR TUNING A MULTITURN INDUCTION HEATING COIL This invention relates to the art. of induction heating and primarily to a method and apparatus for tuninga multi-tum induction heating coil used for induction heating.

The invention is particularly applicable for use in tuning a multi-turn induction heating coil of the type used for heating workpieces as they pass through the central workpiece receiving passageway of the coil and it will be described with particular reference thereto; however, it should be appreciated that the invention has much broader applications and maybe used for tuning various other induction heating. coils.

It is common practice to heat workpieces, such as forging billets, by progressively moving them through the central workpiece receiving passageway of a multi-tum induction heating coil. This coil is connected across the output leads of an alternating current power supply by a conductor arrangement which generally includes two closely spaced bus bars which extend to the coil and two conductors extending in opposite directions from the respective bus bars to the ends of the coil. A capacitor is included in the output circuit of the power supply to adjust the power factor of the installation. This capacitor is used to create inductive reactance which is balanced with the inductive reactance of the coil for tuning the total installation. The inductive reactance of the coil itself varies with the workpiece being heated. Consequently, the actual use of the coil must be considered when designing a coil for a particular use. The coupling between the leads or conductors adjacent the coil also affects the inductive reactance of the assembly including the coil and these conductors. This variable is not generally affected by the workpiece being heated; however, it is affected by the existence of ferromagnetic material close to the conductors.

The procedure for calculating the design of the assembly including the coil and the two leads or conductors extending along one side of the coil involves the calculation of the expected parameters which will affect the total inductive reactance of this assembly. It is'not too difficult to compute the inductive reactance caused by the coil itself, taking into consideration the workpieces which are to be heated; however, it is difficult to calculate the effect of the leads extending along the side of the coil. These leads carry current which creates a mutual inductance between these leads and the coil proper. This mutual inductance varies substantially by the particular surroundings in which the assembly is to be used which surroundings can not be accurately determined when designing the spacing of these leads from the coil. Consequently, often coil assemblies including the closely spaced leads have not operated satisfactorily and require a substantial re-engineering.

The present invention is directed toward a method and a design of the coil and coil leads extending along the side thereof, which method and design can be used to tune the multi-tum induction heating coil installation with the power supply at the actual heating installation. This greatly simplifies the engineering difiiculties in providing a properly tuned installation.

In accordance with the present invention, there is provided an improvement in an induction heating coil including two input leads extending axially along the length thereof, which improvement is the incorporation of means for adjustably shifting the conductors with respect to the coil periphery to change the mutual reactance between the conductors and the coil.

In accordance with another aspect of the present invention there is provided a method for tuning a multi-turn induction heating coil, which method includes adjusting the spacing between the input leads extending axially along the coil and the coil itself after the coil has been placed into the induction heating installation.

The primary object of the present invention is the provision of a method and apparatus for tuning a multi-turn induction heating coil, which method and apparatus may be employed after the coil is placed in the induction heating installation.

Another object of the present invention is the provision of an induction heating coil having two axially extending input leads which coil incorporates means for adjusting the spacing between the coil and the axially extending leads.

These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings in which:

FIG. 1 is a side plan view showing, somewhat schematically, the preferred embodiment of the present invention;

FIG. 2 is an end elevational view of the structure shown in FIG. 1;

FIG. 3 is a top elevational view of the structure shown in FIG. 1;

FIG. 4 is a partial cross-sectional view taken generally along line 4-4 of FIG. 1;

FIG. 5 is a wiring diagram showing, somewhat schematically, the operation of the present invention;

FIG. 6 is a side elevational view similar to FIG. 2 illustrating a modification of the present invention;

FIG. 7 is a partial schematic view illustrating the operating characteristics of the preferred embodiment of the invention shown in FIGS. 1-4; and,

FIG. 8 is a partial schematic view illustrating an operating characteristic of the embodiment of the invention illustrated in FIG. 6.

Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiments of the invention only and not for the purpose of limiting same, FIGS. 1-4 show an induction heating installation A including a multi-tum induction heating coil B having an internal workpiece receiving passage 10, a series of turns 12, spaced terminal ends l4, l6 and a central axis a. An alternating current power supply, schematically illustrated as generator 20, is used for energizing the coil B, in accordance with known induction heating practices. The power factor of the heating installation is corrected by an appropriate capacitor 22 connected between bus bars 24, 26, which are relatively fixed with respect to the power supply 20. In practice, the bus bars 24, 26 are electrically connected onto the coil B by relatively rigid conductors extending between the ends l4, l6 and the bus bars. These conductors are formed as a unit with the induction heating coil to provide an integral assembly.

In the illustrated embodiment of the invention shown in FIGS. 1-4, the conductors extending between the bus bars 24,26 and the coil ends l4, 16 are generally L-shaped conductors 30, 32. These conductors are substantially identical; therefore, conductor 32 will be described in detail, and this description will apply equally to the conductor 30. Two generally orthogonal portions, i.e., axially extending element 40 and radially extending element 42, form the primary elements of the conductor 32.

Referring now briefly to FIG. 7 the operating characteristic of the installation illustrated in FIGS. 1-4 is schematically illustrated. Current flow through the element 40 creates a field b, while at the same time the current flow through the turn 12 of the coil B creates a series of fields c which are generally orthogonal to the field b. The interaction between these intersecting fields controls the mutual inductance and, thus, controls the voltage drop along the lead or element 40. External ferromagnetic structurescan also affect these fields and, thus, change the mutual inductance caused by the interaction of these fields. The present invention relates to a structure and method which utilizes the principle illustrated in FIG. 7 as a means for tuning the complete heating circuit of the induction heating installation. The spacing x between the turns I2 and element 40 directly affects the level of mutual inductance between these elements. In accordance with the present invention, there is provided means for actually adjusting the spacing x after the coil has been positioned in the induction heating device. In this manner, the mutual inductance between the element 40 and the turns 12 can be adjusted to change the inductive reactance in the heating circuit and tune the heating circuit with respect to the capacitor 22 in the output circuit of generator 20. Although a variety of structures could be used for adjusting the spacing x, in accordance with the preferred embodiment of the invention, there is provided a means for shifting the conductor 40 radially with respect to the coil turns 12. This function could be accomplished by a variety of structural details. In accordance with the illustrated embodiment of the invention, the means for adjusting the spacing x with respect to the conductor 32 will be described in detail. This description, of course, applies equally to the adjustment of the conductor 30 with respect to the turns 12 at the opposite end 14 of the coil B. The support plate 50 is secured onto coil adjacent end 16, and this plate is provided with an elongated opening 52 extending generally radially from the axis a. Leg 54 is an integral part of conductor 32 and is provided with a plate 56 onto which is mounted an outwardly extending stud 60. This stud extends through elongated opening 52 and threadably receives an appropriate fastening device, such as nut 62. By using this structure, the coil B could be moved away from the conductor 32 to change the mutual inductance between the element 40 and the coil turns 12; however, this concept is not employed in the illustrated embodiment of the invention. It has been found that the position of the coil B is generally fixed with respect to other auxiliary equipment, so that it is not practical to change the actual position of the coil. In view of this, it would be possible to shift the conductor 32 away from the coil to change the mutual inductance at element 40. This is the concept illustrated.

To allow such movement, without moving the power supply 20, an adjustable connection is required between element 42 and bus bars 24, 26. A variety of arrangements could be used for this adjustable connection; however, in accordance with the preferred embodiment of the present invention, adjustable couplings 70, 72 are provided. These couplings are substantially identical; therefore, only coupling 70 will be described in detail. This description will apply equally to the coupling 72. A plate 80 is fixed upon bus bar 26 and is provided with an elongated hole or opening 82 extending generally radial of the axis a, as best shown in FIG. 2. Plate 82 is secured onto element 42 and carries an outwardly extending stud 86 protruding through opening 82. An appropriate fastening device, such as nut 88 is threadably received upon stud 86 for clamping the coupling 70 into a variety of adjusted positions.

After the coil B has been secured onto bus bars 24, 26 and the appropriate capacitor 22 has been incorporated, the electrical characteristics of the coil are determined for use with a selected workpiece. Thereafter, the nuts 62, 88 are loosened and the conductors 30, 32 are shifted with respect to the coil until the circuit of the induction heating installation A is properly tuned. Thereafter, the nuts 62, 88 are tightened and the induction heating installation is ready for service. Of course, several adjustments may be necessary before the final tuning has been accomplished.

FIG. 5 is a schematic wiring diagram to illustrate, somewhat briefly, the operating characteristics of the invention so far described. Capacitor 22 has a capacitive reactance X The leads or conductors 30, 32 have inductive reactances X X and resistances R and R respectively. The coil itself has an effective resistance R,, and an effective inductive reactance X,,,,. By adjusting the spacing between the elements 40 and the turns 12, as shown in FIG. 7, the mutual inductance between these elements is varied. This is illustrated as an adjustment of the inductive reactances X and X in FIG. 5. In this manner, the circuit shown in this figure can be tuned after the coil has been properly positioned for heating.

A majority of the induction heating installations requires a cooling system. A variety of cooling systems could be used; however, as illustrated in FIGS. 14, the bus bars 24, 26 are hollow and coolant is pumped therethrough. Coolant is circulated into element 42, through element 40 and out element 54. To cool the coil B, the coil is formed from a hollow conductor having an inlet 90 and an outlet 92. In this manner, the various elements of the installation are cooled by circulation of a coolant therethrough.

Referring now to FIG. 6, a modification of the present invention is illustrated. In accordance with this modification, the conductor 32 is moved in a direction generally normal to plane y extending through axis a. This is substantially perpendicular to the adjustment described in FIGS. 1-4; however, the same basic result is obtained, i.e., the mutual inductance between the elements 40 and the turns 12 is changed by varying the effect between the fields b, c. In this embodiment, like parts have the same numbers as the parts in FIGS. 1-4. A plate has an opening 102 extending generally perpendicular to leg 54. In this manner, the conductor 32 may be moved in the direction of the elongated opening 102. A plate is secured onto element 42 and has an opening 112 extending perpendicular to element 42. Referring now to FIG. 8, when the nuts 62, 88 are loosened, the elements 40 may be moved in the direction of the arrow to change the mutual inductance between the fields b, c. This has the same effect in tuning the heating circuit as described in detail above. Other arrangements could be used for changing the distance between the element 40 and the turns 12. Elongated holes extending in various directions could be employed to accomplish this change in spacing. It is also possible to enlarge the width of the element 40 to change the mutual inductance between this element and the turns of the coil.

Having thus described my invention, I claim:

1. In an induction heating device including a multi-tum induction heating coil having a central axis, an outer periphery and spaced ends, a first conductor connected onto one of said ends and extending axially of said coil and adjacent said periphery, and a second conductor connected onto the other of said ends, said conductors being adapted for connection to an alternating current power supply means for energizing said coil whereby current flow in said first conductor and said coil results in a mutual inductance between said first conductor and said coil and affects the reactance of said first conductor, the improvement comprising: means for adjustably shifting said first conductor with respect to said coil periphery to change the. mutual inductance between said first conductor and said coil.

2. The improvement as defined in claim 1 wherein said adjustable shifting means includes means for shifting said first conductor in a direction generally radial of said coil.

3. The improvement as defined in claim 1 wherein said adjustable shifting means includes means for shifting said first conductor in a direction generally normal to a plane defined by the axis of said coil.

4. In an induction heating installation including a multi-tum induction heating coil having several turns, a central axis and spaced ends, a pair of leads, each of which is electrically connected adjacent one of said spaced ends of said coil, a source of alternating current having two generally fixed output bus, means for connecting one of said leads to each of said generally fixed output bus, wherein said leads extend generally axially of said coil and adjacent said coil and whereby the fields around said leads are inductively coupled with the fields around the turns of said coil, the improvement comprising: means for adjusting the spacing between said leads and said coil to vary the inductive coupling between said leads and said coil turns.

5. In an induction heating device including a multi-tum induction heating coil having a central axis, an outer periphery and spaced ends, a first conductor connected onto one of said ends and extending axially of said coil and adjacent said periphery, a second conductor connected onto the other of said ends, said conductors being adapted for connection to an alternating current power supply means with two output leads for energizing said coil whereby current flow in said first conductor and said coil results in a mutual inductance between said first conductor and said coil and affects the reactance of said first conductor and a power factor correcting capacitor connected across said output leads, the improvement comprising: means for adjusting the mutual reactance between said first conductor and said coil to tune said coil with said capacitor.

6. A method of tuning a multi-tum induction heating coil including at least one lead extending axial of said coil and closely spaced therefrom, said method comprises the steps of:

a. connecting said coil onto a power supply station having a power factor correcting capacitor and output bus bars; 5 and,

b. adjusting the spacing between said lead and said coil to tune said coil and lead with said power supply station.

i ll 

1. In an induction heating device including a multi-turn induction heating coil having a central axis, an outer periphery and spaced ends, a first conductor connected onto one of said ends and extending axially of said coil and adjacent said periphery, and a second conductor connected onto the other of said ends, said conductors being adapted for connection to an alternating current power supply means for energizing said coil whereby current flow in said first conductor and said coil results in a mutual inductance between said first conductor and said coil and affects the reactance of said first conductor, the improvement comprising: means for adjustably shifting said first conductor with respect to said coil periphery to change the mutual inductance between said firsT conductor and said coil.
 2. The improvement as defined in claim 1 wherein said adjustable shifting means includes means for shifting said first conductor in a direction generally radial of said coil.
 3. The improvement as defined in claim 1 wherein said adjustable shifting means includes means for shifting said first conductor in a direction generally normal to a plane defined by the axis of said coil.
 4. In an induction heating installation including a multi-turn induction heating coil having several turns, a central axis and spaced ends, a pair of leads, each of which is electrically connected adjacent one of said spaced ends of said coil, a source of alternating current having two generally fixed output bus, means for connecting one of said leads to each of said generally fixed output bus, wherein said leads extend generally axially of said coil and adjacent said coil and whereby the fields around said leads are inductively coupled with the fields around the turns of said coil, the improvement comprising: means for adjusting the spacing between said leads and said coil to vary the inductive coupling between said leads and said coil turns.
 5. In an induction heating device including a multi-turn induction heating coil having a central axis, an outer periphery and spaced ends, a first conductor connected onto one of said ends and extending axially of said coil and adjacent said periphery, a second conductor connected onto the other of said ends, said conductors being adapted for connection to an alternating current power supply means with two output leads for energizing said coil whereby current flow in said first conductor and said coil results in a mutual inductance between said first conductor and said coil and affects the reactance of said first conductor and a power factor correcting capacitor connected across said output leads, the improvement comprising: means for adjusting the mutual reactance between said first conductor and said coil to tune said coil with said capacitor.
 6. A method of tuning a multi-turn induction heating coil including at least one lead extending axial of said coil and closely spaced therefrom, said method comprises the steps of: a. connecting said coil onto a power supply station having a power factor correcting capacitor and output bus bars; and, b. adjusting the spacing between said lead and said coil to tune said coil and lead with said power supply station. 